Publications are referenced throughout the specification in parenthesis. Full citation corresponding to each reference is listed following the detailed description. The disclosures of these publications are herein incorporated by reference in their entireties in order to describe fully and clearly the state of the art to which this invention pertains.
Molecules absorbing, emitting, or scattering light in the visible, near-infra red (NIR), or long-wavelength (UV-A, >300 nm) region of the electromagnetic spectrum are useful for optical tomography, optical coherence tomography, fluorescence endoscopy, photoacoustic technology, sonofluorescence technology, light scattering technology, laser assisted guided surgery (LAGS), and phototherapy. The high sensitivity associated with fluorescence phenomenon parallels that of nuclear medicine, and permits visualization of organs and tissues without the negative effects of ionizing radiation. Targeted delivery to a particular site in the body of diagnostic and therapeutic agents (generally referred to as “haptens,” “effectors,” or “functional units”), such as fluorophores, photosensitizers, radionuclides, paramagnetic agents, and the like, continues to be of considerable demand in diagnosis, prognosis, and therapy of various lesions (Hassan et al., Licha et al., Shah et al., Vasquez et al., and Solban et al.). The conventional targeting method, referred to as “bioconjugate approach” or “pendant design” involves chemical attachment of these agents to bioactive carriers which target a particular site in the body. In the bioconjugate approach, the two units can exist and function independently wherein the functions of targeting and imaging/therapy may be separable. Bioactive carriers include small molecule drugs, hormones, peptidomimetics, enzyme inhibitors, receptor binders, receptor antagonists, receptor agonists, receptor modulators, DNA binders, transcription factors, inhibitors of the cell cycle machinery, transduction molecules, inhibitors of protein-protein interactions, inhibitors of protein-biomacromolecule interactions, macromolecular proteins, polysaccharides, polynucleotides, and the like. The bioconjugate approach has been explored extensively over the past several decades, and has met with moderate success, particularly in tumor detection, when medium and large size carriers (c.a. molecular weight >1000 Daltons) are employed (Licha et al. and Shah et al.). This is because attachment of dyes, drugs, metal complexes, or other effector molecules to macromolecular carriers such as antibodies, antibody fragments, or large peptides does not greatly alter the bioactive targeting properties; i.e., the bioconjugate is still able to bind to the receptor effectively. However, this approach does have some serious limitations in that the diffusion of high molecular weight bioconjugates to tumor cells is highly unfavorable, and is further complicated by the net positive pressure in solid tumors (Jain et al.). Furthermore, many dyes tend to form aggregates in aqueous media that lead to fluorescence quenching.
A need therefore exists for photoactive small molecules that also have bioactive targeting capabilities. However, a problem in designing small molecule bioconjugates is that the binding of a diagnostic or therapeutic agent to a targeted receptor is often observed to be severely compromised when the sizes of the diagnostic or therapeutic agent and the bioactive targeting carrier are similar (Hunter et al.). Thus, substituting a large functional unit such as a dye or a photosensitizer into small molecule drugs, presents a formidable challenge. In order to overcome this problem, methods (referred to as “integrated approach” or “internal bifunctional approach”) have been practiced wherein a radionuclide metal ion is incorporated into a steroid or morphine alkaloid framework such that the molecular topology of the original drug and the corresponding radionuclide mimic are very similar (Rajagopalan, U.S. Pat. No. 5,330,737; Rajagopalan, U.S. Pat. No. 5,602,236, and Hom et al.). In contrast to the bioconjugate approach described above, both functions of the integrated unit (e.g., targeting and imaging/therapy) are inseparable. The integrated approach is based on the principle that antibodies, enzymes, and receptors are multispecific and will bind to any molecule that is topologically similar to a natural antigen, substrate, or ligand. Previous work on steroid mimics confirm that integrating a metal ion into natural receptor ligands is a viable strategy for selective delivery of diagnostically and therapeutically useful radionuclides to target tissues (Hom, et al. and Skaddan et al.). This integrated design incorporates a single-atom isosteric substitution of a functional unit into a molecular framework. However, substituting a large functional unit such as a dye or a photosensitizer into small molecule drugs, peptides, pseudopeptides, or peptidomimetics presents a formidable challenge. While transformation of a nucleoside to fluorescent nucleoside has been previously reported (Miyata et. al.), the peak electronic spectra (absorption, excitation, and emission) remained in the UV region. In addition, this transformation is limited to this single nucleoside use.